Control of viscosity and polycaproamide degradation during vacuum polycondensation

ABSTRACT

CONTROL OF POLYCAPROAMIDE DEGRADATION DDURING VACUUM POLYCONDENSATION IN A POLYMER FINISHER IS ACHIEVED BY MANTAINING A PARTIAL PRESSURE OF WATER ABOVE THE POLYMER MELT ABOVE ABOUT 10 MM. HG, PREFERABLY ABOVE ABOUT 30 MM. HG, ABSOLUTE PRESSURE. DEGRADATION IS MINIMIZED TO WITHIN 3, PREFERABLY 2 UNITS OF THE THEORETICAL DIFFERENCE BETWEEN CARBOXYL AND AMINE ENDS. THE WATER EXTRACTABLES CONTENT OF THE POLYMER IS ALSO CONTROLLED TO BELOW ABOUT 3.5% BY WEIGHT, PREFERABLY 2.5%, VISCCOSITY INCREASE OF THE POLYMER MELT LEVELS OUT AFTER LESS THAN 4 HOURS TO LESS THAN 10 FAV UNITS PER HOUR, BY MEANS OF THE INVENTION.

United States Patent Office 3,753,955 Patented Aug. 21, 1973 U.S. Cl. 260-78 L 3 Claims ABSTRACT OF THE DISCLOSURE Control of polycaproamide degradation during vacuum polycondensation in a polymer finisher is achieved by maintaining a partial pressure of water above the polymer melt above about mm. Hg, preferably above about 30 mm. Hg, absolute pressure. Degradation is minimized to within 3, preferably 2 units of the theoretical dif' ference between carboxyl and amine ends. The water extractables content of the polymer is also controlled to below about 3.5% by weight, preferably 2.5%; viscosity increase of the polymer melt levels out after less than 4 hours to less than 10 FAV units per hour, by means of the invention.

BACKGROUND OF THE INVENTION This invention relates to a process for the polymerization of caprolactam or amino caproic acid. More specifically, this invention relates to a method to control thermal degradation and viscosity increase while removing extractables in a polymer finisher under vacuum.

The prior art patents, of which we are aware, considered to be most pertinent to this invention are U.S. 2,904,109 to Malm, U.S. 2,731,081 to Mayner, U.S. 3,578,640 to Twilley, Coli, IL, and Roth, J. and U.S. 3,526,484 to Kilpatrick. The pertinent portions of all the above patents are hereby incorporated by reference.

The Malm and Mayner patents teach removing monomers from a polycaproamide melt by sweeping with steam prior to spinning. Kilpatrick and Twilley et al. teach direct spinning of nascent polymer but the monomer and other water extractables are removed by vacuum venting. These latter patents teach use of a polymer finisher (labeled 60 7 in the Twilley figure and labeled 8 in the Kilpatrick figures) but do not teach maintaining high partial pressure of steam in the finisher. Nor do these references teach control of polymer viscosity, or even recognize a polymer degradation problem in the finisher.

SUMMARY OF THE INVENTION Control of polymer degradation and rate of polymer viscosity increase during the predominantly polycondensation reaction in the polymerization of polycaproamide in a polymer finisher at a temperature between about 225 C., and about 300 C. at an absolute pressure of from about 11, preferably 31 mm. Hg, to about 150 mm.

Hg is achieved by sweeping the finisher with steam, the

polymer melt preferably having a high surface area. The partial pressure of the steam in the vapor above the melt must be maintained above about 10 mm. Hg, preferably about 30 mm. Hg, to minimize thermal degradation of the polymer in the melt to within 3 units (at above 30 mm. Hg within 2 units) of the theoretical difference between amine ends and carboxyl ends. Also, the rate of polymer viscosity increase of the polymer melt will level out after less than 4 hours, preferably less than 3 hours, to less than 10 FAV units per hour by means of this steam purge in the finisher. This control of degradation and rate of viscosity increase is achieved without any penalty to extractables content. The water extractables in the spun polycaproamide can still be kept at below 3.5 preferably below 2.5% by weight, even with the control of degradation and rate of viscosity increase. The preferred method to maintain this partial pressure of steam is by purging or sweeping the polymer finisher with steam overhead and counter to the flow of the polymer melt.

DESCRIPTION OF PREFERRED EMBODIMENTS Apparatus to carry out the process of this invention is shown in the figure of U.S. 3,578,640 with the exception that the vacuum line 64 would be located at the polymer inlet end of the finisher and a steam line would be installed at the polymer exit end of the finisher where vacuum line 64 is presently shown. Thus, steam would sweep overhead of the polymer melt in finisher 60, counter to the flow of the polymer melt. A more preferred polymer finisher, because it provides higher surface area to the polymer melt would be the finisher 8 in FIGS. 1, 2, and-5 of U.S. 3,526,484 equipped with a steam line and vacuum line to provide counter flow steam sweeping as described above.

A description of the method of determining the carboxyl groups or ends and amine groups or ends, the formic acid relative viscosity (FAV), the water extractables and the monomer content of the polycaproamide polymers prepared in the examples following is found in U.S. 3,578,640, Twilley et al., column 5, lines 24 to 47. The difference of ends or A ends is simply the arithmetic difference of the analytical determination of ends of carboxyls and the analytical determination of ends of amine. Amine ends are depicted [NH carboxyl ends [COOH] and polymer is depicted [NHCO].

Previous work has shown that the presence of a phosphorous containing light-stabilizer causes considerable catalysis of polycondensation under vacuum. Although this reduces the polycondensation time and thus potentially increases the throughput of the polymer train, the viscosity becomes extremely diflicult to control; a small difference in residence time causes a large difference in viscosity of the polymer melt. The presence of water in the polymer melt determines its viscosity since the viscosity stabilizes when the product of ends is in equilibrium with the water content.

d(FAV) dt Thus, an increase in the water content reduces both the rate of approach to equilibrium and the formic acid viscosity at this equilibrium point.

It would appear obvious, therefore, that to stabilize ak [NH [COOH]-4:1 [NHCO] [H O] the viscosity at a required value, the polymerization should be run at such a pressure that the partial pressure of the water in the vapor is in equilibrium with the water in the melt corresponding to the required viscosity. However, owing to the difference in boiling points and concentration of water and lactam, the vapor above a polycaproamide polymer melt is composed mainly of lactam and an increase in the partial pressure of water leads to a corresponding increase in the vapor pressure of lactam. The lactam in the polymer melt then exceeds the level that can be tolerated for a direct spin process.

In order to separate these two apparently interdependent factors, it was decided to investigate the possibilites of using a steam sweep across the polymer melt. A high partial pressure of water is then obtained, determined 3.753.955. 3 I I r t 4 by the absolute steam pressure above thepolymer melh, ..sure,, le adsto ,a further lowering of the monomer and while a low partial pressure of lactam arises from the extractables level (see Table II and FIG. 1). This sugsweeping action of the steam. The steam pressure, theregests that the polymer melt extra'ctables level is surface fore, 60113015 the equilibrium Viscosity and P Y generation dependent, and the presence of a steam sweep tion rate p f ly f the eXtmctableS level of t in a reactor specifically designed for high surface generapolynflel' melt Whlch confironed by E sweep Fa tion, such as the squirrel-cage finisher designed for the Thls approach was mvestlgated first m a research size direct-spin process, may give even lower extractables levels reactor by conducting the Polycondensanon. at at the above pressures than those recorded with Research pressures between 5 mm. and 100 mm. and Introducing water into the ol mer melt at a constant rate durin" f the final 2 /2 hoiirs of the 3-hour polycondensation time Thermal degradatlon The generated steam sweep was varied by changing the u 0 water/polymer melt ratio and runs at difierent pressures Sea1ed i reported m thls mvestlgaftlon Show a were compared with runs at the same pressure in the change 111 the dlfififence (it ends col'l'espondmg to a absence of steam. The first results, tabulated below, are carboxylatlon Teactlon- However, the Presence of Steam concerned with a medium-d ein polymer using ti over the polymer melt and the consequent increase in the acid and amino-propylmorpholine as terminators. water content of the melt appears to afiord some protec- TABLE L-EFFECT 0F PRESSURE AND STEAMSWEEP ON A. MEDIUM'DYEING (B) POLYMER USING 100 00's WATER IN 2% HOURS, T.=260 C.

FAV Extractables' Lactam Sealed Steam Sealed Steam Sealed Steam Pressure 0 hrs. 3 hrs. 0 hrs. 8 hrs. 0 hrs. 3 hrs. 0 hrs. 3 hrs. 0 hrs. 3 hrs. 0 hrs. 3 hrs.

TABLE II.- EFFECT or GENERA'IED STEAM SWEEP AT CONSTANT PRESSURE ON A MEDIUM DYEING POLYMER (B) T.=260 C.

Extraetables, Lactam, FAV percent percent Ohrs. 3111's. Ohrs. 3hrs. Ohrs. 3 hrs.

(i) Pressure=1flo mm;

Conditions:

Sealed/800 g. polymer 26. 9 37. 2 9. 78 9. 13 7. 16 6. 59 100 cc. HzO/8U0 g. po1ymer 27. 4 45. 4 10. 4. 87 7. 72 2. 84 200 cc. 11101800 g. polymer- 21.3 40. 9 10. 04 3. 88 7.87 2:08 200 cc. B 0140!) g. polymer 24. 1 43. 5 9. 18 2.57 6. 67 0. 90

(ii) Pressure=50 mm.

Conditions:

Sealed/800g. polymer 25. 3 44. 4 9. 4. 18 8:14 2. 62 Sealed/800 g. polymer--. 22. 6 61. 3 8. 91 4. 66 6. 89 2. 40 100 cc HzO/SGOg polymer 29.5 48.8 9. 26 2. 7. 34 0.92 200 cc Ego/s00 g polymer 26.6 75.9 8. 92 2.37 6. 50 0.68 7 liter N zlhrx/SDO g. polymer 22. 7 85. 2 8. 82 1. 94 6. 0. 59

1 Polymer contains manganese hypophosphite.

Similar experiments were carried out with a light-dyetion'irom this degradation reaction for the loss in carboxyl ing polymer and are tabulated below. end groups is considerably reduced.

TABLE Ill-EFFECT 0F PRESSURE AND STEAM SWEEP ON A LIGHT-DYEING POLYMER USING 200 ecs WATER IN 2% H118. T.= 260 C.

FAV Extractables Lactam Sealed Steam Sealed Steam 1 Sealed Steam Pressure 0 hrs. 3 hrs. 0 hrs. 3 hrs. 0 hrs. 3 hrs. '0 hrs. 3 hrs. (this. 3 hrs. 0 hrs. 3 hrs.

5 mm. 24. 4 150. 7 29. 8 123. 4 9.76 2. 13 8. 1. 83 6. 25 0. 29 6. 67 0. 11 107. 9 28. 0 111. 4 9. 71 4. 61 9. 92 2. 46 '6. 92 2. 49 6. 55 0. 48 75. l 30. 9 104. 2 9. 41 8. 18 9. 56 2. 78 7. 37 6. 49 6. 74 0. 88

l Polymer contains manganese hypophosphite.

Extractables and m n m level 5 TABLE- rv. nnnncr OF STEAM SWEEP 0N END -GR OUP LEVEL OF A MEDIUM-DYEING POLYMER It 18 ObVlOllS from the above data that a steam sweep USING 200 ccs WATER IN 2 HOURS. T.:260 c. .(POLYMER LABELED 'B) through the polymer melt considerably reduces its monomer content and, therefore, its extractables level. It also 2%???233? 1 35 5 53? appears that, at a given volume of steam, the monomer in 70 (NHz-COOH) (NHPCQOED Starting material 27. 4 the melt 1s dependent on the steam pressure (see Table 5mm. hrs n 33.5 352 1). Of greater srgnrficance, however, is the demonstrated $8 52- 38.; g: g fact that an increase in the volume of steam per gm. of 1 "III: 3510 26 1 2s 7 mmJB hrs 35. 7

polymer swept through the melt, at constant steam pres TABLE V.EFFECT OF GENERATED STEAM SWEEP AT CONSTANT PRESSURE ON THE END GROUP LEVEL OF Sealed/800 g. polymer/3 hrs 100 cc.s Ego/800 g. polymer/3 h 200 ccs H2O/800 g. polymer/3 hrs. 200 ccs Bro/400 g. polymer/3 hrs.--

(ii) Pressure=50 mm.

southw- Conditions:

( Starting mfiteriaL 29.8 Sealed/800 g. 3 hrs 35. 1 100 cc.sH20 800 g./3 hrs 26. 3

Difference of ends OH-NH2) (11) Starting material.

Sealed/800 g./3 hrs. 5 2. 8 1.4 200 ce.'s HzO/SOO g.l3 hrs. 4. 1 4. 4 7 liters N2/hr./800 g./3 hrs. -0. 2 4. 6

l Polymer contains manganese hypophosphite.

BLE TL-EFFECT OF PRESSURE AND STEAM SWEEP ON THE END GROUP LEVEL OF A LIGHT-DYEING POLYMER USING 200 cc.s WATER IN 2% HRS. T. =260 O.

Sealed-Difierenee of Steam-Difierence of ends (CODE-N112) ends (COOH-NHz) Starting material 3 hrs.

6 hrs 6 hrs.

The data reported is consistent in that polymerizations conducted in the presence of a steam sweep show a much smaller loss in carboxyl groups compared to the corresponding pressure of a sealed system. It is interesting to note that effect is less pronounced at lower pressures-1n a medium-dyeing polymer no significant protection is obtained below 20 mm. Above this pressure the difference of ends increases with decarboxylation in the sealed system but is unchanged in the presence of steam. (Tables IV and V.) The light-dyeing polymer, having a higher carboxyl end group content, would be expected to be more susceptible to decarboxylation and the difference between the carboxyl and amine group levels, therefore, decreases through decarboxylation. Again, the presence of steam protects the polymer, although the efiect is less marked at lower pressure (Table VI).

It would appear from the data reported that the decarboxylation reaction is not dependent on the presence of phosphorous containing light stabilizers, but also occurs in a non-catalyzed polymer melt. The fact that the presence of staem significantly retards the decarboxylation suggests that water in the polymer melt is an important part of the decarboxylation mechanism and is involved in the rate-determining step. The exact mechanism of decarboxylation is not known, but a reaction between two carboxyl groups is possible.

Although this is possible during the early stages of I polymerization under vacuum and in an acid-terminated polymerization where there is a large excess of carboxyl groups, it becomes progressively less likely as the polymerization proceeds and the number of carboxyl end groups decreases.

A second mechanism has been suggested by Dr. H. Reimschuessel involving attack by the carboxyl groups in the polymer backbone amide bonds.

Such reaction results in the loss of a carboxyl group and the gain of an amine since the Schifi base (I) will titrate as an amine. The gain in amines will not be exactly equivalent to the loss in carboxyl groups, however, since there is the possibility of reaction to the imide (II) which does not titrate as an amine. This loss in carboxyl groups and corresponding gain in amines has been noticed previously, and the reaction becomes more likely as the polymer viscosity increases since the mobility of the end groups decreases and the statistical chance of such a reaction increases at the expense of the carboxylamine amide condensation.

Extractables and monomer level Trials were carried out in a pilot plant reactor at varying pressures from 5-90 mm. and at different steam flow rates. Table VII summarizes the conditions necessary for polymerization without steam.

TABLE VIL-EFFECT OF PRESSURE ON A LIGHT-DYEING POLYMER Without the presence of the steam sweep a low pressure of mm. is required for stripping of the excess monomer from the polymer melt to give an extractables level below 2.5%. The high degree of termination (60 equivalents of acetic acid) and the low temperature (245 C.) are required to give a polycondensation time of 2 hours. The difference of ends (COOHNH =44) is smaller than the theoretical value of 60, which corresponds to the amount of terminator added.

Table VIII demonstrates the etfect of steam on the reaction rate, extractables level and degradation. The steam flow rates, termination levels and pressure were varied and in each case the viscosity (estimated from the kw. reading of the agitator) rose rapidly to an equilibrium value, then remained constant at this value. These results are plotted graphically in FIG. 2. The heavy, black line 3 corresponds to polymerization at .5 mm. without steam.

mer was taken through the heated gate valve which was maintained at the same temperature as the polymer by a separate heater. Vacuum was then applied to the agitated system and the internal pressure decreased to the required operating value over a period of /2 hour. Water was then pulled into the reactor through the nitrogen purge tube which reached to the bottom of the polymer melt. The rate of water. addition was controlled by a needle valve over a period of 3 hours. After 3 hours, under vacuum, a sample was taken through the gate valve and the reactor then pressurized with nitrogen and sealed at p.s.i.g. The polymer was maintained at this temperature and pressure for a further 3 hours, taking samples every hour. Experiments were repeated, samples analyzed in duplicate and the average of the results used.

Low molecular weight polymers used (A) Light-dyeing polymer: 26 equivalents/1t) g. of

TABLE VIH.'EFFECT OF PRESSURE AND STEAM SWEEP ON A LIGHT-DYEING POLYMER Vacuum conditions Pres- Percent Batch sure, M1. water/ Temp., extraet- Theoretical size, mm. mm. Time C. ables (GOOH-NH) (COOH-NH) lbs.

70 4. 58 244 47 4. 51 54 550 50 50 3. 42 244 55 3. 7 '18 50 550 60 60 3. 72 246 52 5. 2 49 50 550 El) 60 2. 42 245 51 3. 5 42 50 275 6D 60 4. 25 246 54 2. 5 46 50 275 90 90 3. 58 246 54 5. 3 36 47 550 95 90 3. 84 259 57 3. 7 41 50 550 45 70 4. 55 250 60 3. 3 44 52 550 Viscosity control Table VIII also shows that an acceptable extractables level of 3.5%, preferably 2.5 was obtained at 60 mm. when the batch size was reduced by half and the steam sweep allowed to continue for 4% hours even after the target viscosity had been reached. This should be compared with an extractables level of 6% obtained at 60 mm. without steam sweep, and confirms the research data showing that surface generation and mass transfer are important in reducing the extractable level under a steam sweep.

From FIG. 2, it can be seen that the presence of the steam sweep enables more efiiective control at the target viscosity-the change in viscosity being of the order of 3-4 units per hour compared to 10-12 units per hour at 5 mm. without steam. Numbers on the curves indicate batch numbers. In Table V-I'II an increase in the mass transfer and surface generation achieved by halving the batch size to 275 lbs. gives an increase in reaction rate and corresponding reduction in polymerization time. This is obtained without loss of viscosity control; for at the target viscosity, the rate of change is of the order of 1 unit per hour.

Thermal degradation The pilot plant data also follows the research data in indicating that the presence of steam over the polymer melt protects it from loss of end groups by decarboxylation. In Table VIH, the difference of ends is much closer to the theoretical value required by the terminator content than is obtained at 5 mm. in the absence of steam (Table VII).

Experimental procedure: Research autoclave Low molecular weight, unwashed polymer chips were fed into a nitrogen-purged autoclave and the reactor sealed under an internal nitrogen pressure of 25 p.s.i.g. External heating was applied until the internal temperature of the polymer, as measured by a thermocouple at the bottom of the hollow-shafted, double spiral agitator, was equal to the, required temperature. A sample of the polyacetic acid termination. 10 p.p.m. manganese as manganous hypophosphite.

Theoretical A Ends 26 FAV: 17; [COOH] =60; 10.9%

[NH =3 l; Extractables:

[ I-I ]=87; Extractables= (C) Medium-dyeing polymer: 2'5 equivalents/1t) g. of acetic acid and 25 equivalents/ 10 g. of cyclohexyl-amine termination. 10 p.p.m. manganese as manganous hypophosphite.

Theoretical A Ends 0 FAV=36; [COOH] =38; [NI-I =33; Extractables: 8.9

EXPERIMENTAL RESULTS: RESEARCH AUTOCLAVE 1. Polymer: A (800 g.) Temperature: 260 0. Pressure: 5 mm. Hg Water added: 200 cc.s in 3 hours COOH, NHz, Extraetables Laetam, Time (hrs.) FAV eqjltl g. eq./10 g. percent percent 2. Polymer: A (800 g.) Temperature: 260 0. Pressure: 50 mm. Hg Water added: 200 cc.'s in 3 hours EXPERIMENTAL RESULTS: RESEARCH AUTOCLAVE EXPERIMENTAL RESULTS: RESEARCH AUTOCLAVE COOH, NHa, Extraotables Lactam, COOH, NHz, Extraetables Lactam, Tlme (hrs.) FAV eq./l g. eqJlO g. percent percent Tlme (hrs.) FAV eq./10 g. eq./10 g. percent percent 12. a, Polymer: B 800 g.) Polymer: A (800 g.) Temperature: 260 0: Temperature: 260 0. Pressure: 100 mm. Hg Pressure: 100 mm. Hg Water added: 200 ce.s in 3 hours Water added: 200 cc.s in 3 hours 0 21 a 4 2 8 9 10 04 7 7 8. 1 37.8 10. o 2. 91 0. so

Polymer: B (400 g.) 33.8 9.7 2.95 0.80 Temperature: 260 C.

aera e: 00.5 n ours 4. Polymer: A (800 g.) 1 o 24.1 49.1 80.0 9.18 e. 61 Temperature: 260 C. 3 43. 5 37- 5 64. 8 2. 57 0. 90 Pressure: 5 mm. Hg Water added: No water added 14 Polymer: 13 (800 g.) 53.31 6:3 3:1? 8: 2 3 Temperature 25. s 11.0 2. 2s 0. 42 aressum: 5 H8 23 2 1O. 4 2 34 0 40 ater added. No water added 22. 4 10. 4 2. 46 0. 41 0-" g g 8 56 0. 64

5. Polymer: A (800 g.) i95 B (800 Temperature: 260 C. 25 m Temperature. 260 0. Pressure. 50 mm. Hg P 10 H Water added: No water added 3% P wgter added 0 22. 1 50. 4 p 78. 4 9. 73 8. 15 33; i 12 3 j; S}, g; g: a 45. 5 32. 5 65.7 2. 24 0. 5s 27. 3 12. 7 4. 82 2. 34 21.1 12. a 4. s2 2. a2

golymerzB (SgtgQc empera ure: V t d I I dd 1 P01 erzAwoog) aera e: owaera e g $j g r o- 21. 2 s2. 0 80.3 10. 41 8.85 Water added: No water added 4 2 7 08 07 :2: 2:: :2: 2. P l 2 B 00 81. 5 31. 0 15. 7 8. 39 6. 55 TgIZ pi$ atur26 C. 86. 3 28. 5 14. 7 8. 25 6. 53 Pressure: 50 mm. Hg 93.1 25. 3 14. 5 7- 8 44 Water added: No water added 0 25. 3 45. 6 76. 8 9. 76 8. 14 3 44. 4 32. 2 67. 3 4. 18 2. 62 7. Polymer: B (800 g.) Temperature: 260 C. 18. Pressure: 5 mm. Hg Polymer: B (800 g.) Water added: 100 cc.s m 3 hours Temperature: 260 C. 4 Pressure: 100 mm. Hg g 22.3 $8. g g-Z 1g- 2; 3-13 Water added: No water added 8. Polymer: B (800 g.) 19 Temperature: 260 C. Pressure: 10 mm. Hg ggg gg g gg% Water added: 100 eels in 3 hours presspurez 50 0 23. o m 8 75. 7 9 67 8' 20 Water added: No water added 3 50. 0 31. 9 62. 0 1. 97 0. 32 8g 2. 40 23. 4 23. 0 4. 79 2. 61 9 21. 3 22. 5 4. 2. 51 Polymer: B (800 g.) 19. 9 21.3 4.49 2. 29 gemperatg'er 260"HC. ressure: mm. g Water added: 100 eels in 3 hours 20.

Polymer: C (800 g.) 0 21. 7 54. 4 79. 7 10. 51 8. 38 0 Temperature: 260 C, 3 46. 1 34. 8 65. 1 2. 62 0.69 Pressure: 50 mm.

Water added: 7 liters nitrogen per hour over 3 hours 21. 1 21. 3 1. 94 0. 59 l B 6; 1s. 5 22. 5 2. 13 0. so Temperature. 260 C. 18 2 20 5 2 36 0 63 Pressure: 50 mm. Hg 15 6 20 1 2 51 0 84 Water added: eels in 3 hours Polymer: C (800 g.) Temperature: 260 C.

11, Pressure: 50 mm. Hg

Polymer: B (800 g.) Water added. 200 cc. s in 3 hours Temperature: 260 C.

26. 6 55. 2 49. 8 8. 92 6. 50 Pressure. 100 mm. Hg Water added: 100 eels in 3 hours 5 92. e 22. 1 11. 2 2K 52 of 1s 27. 4 42. 6 71. 0 9. 56 45. 4 31- 7 60 4 4 87 99. 9 20. 2 15. 8 2. 46 0. 83

TABLE IX.VACUUM DEGRADATION CONTROL Polymer deg- Total Steam radation, devi- Data from experimental results pressure, pressure, ation from thein table mm. Hg mm. Hg oretlcal A ends In tests 14 through "18 the system was dehydrated resulting in severe polymer degradation.

In tests 7 through 11, water was added to the system to maintain varying partial pressures. The degradation decreased as the steam press was increased.

In tests 12 and 13, the water addition rate was increased to give higher steam partial pressure, which further reduced the degradation.

Conclusion: The partial pressure of water vapor of over about 10 mm. Hg or preferably over about 30 mm. Hg in the vapor over a polymer melt will greatly reduce the degradation of the polymer.

FIG. 1 and Table X, using data from the Experimental Results as shown, will show the effect of steam stripping on polymer extractable levels. Note that by using steam purge or sweep, extractables can be kept below 3.5%, preferably 2.5% by weight without resorting to the extremely low pressures of the prior art.

Sealed 1 100 cc. H20 1 Percent Percent extract- Percent extract- Percent Pressure ables lactam ables lactam Starting material 9. 98 7. 43 9. 98 7.43 5 mm 2. 33 0. 64 2. 45 0. 42 10 mm- 2. 24 58 1. 97 0. 31 20 mm. 3. 09 1. 07 2. 62 0. 69 50 mm- 4. l8 2. 62 2. 71 0. 93 100 mm. 9. 13 6. 59 4. 87 2. 84 100 rum/200 ec 3. S4 2. 08 100 mm./200 ecu/400 g 2. 57 0. 90

1 800 gram charge, no manganous hypophosphite.

What is claimed is: 1. In a process for the production of polycaproamide shaped articles from molten anhydrous nascent polymer comprising:

(a) continuously polymerizing e-caprolactam at an elevated pressure and temperature in the presence of small amounts of water and a terminating agent to form a prepolymer melt, then (b) continuously removing most of the water and part of unreacted lactam from said prepolymer melt by exposure to vacuum in a vessel providing said prepolymer melt with large surface area-to-volume ratio, residence time in said vessel being about 10 minutes, then (c) continuously transferring said melt to a surface renewal device also having large surface area-tovolume ratio, which promotes completion of the polymerization reaction by polycondensation, and removes residual amounts of vaporiza-ble materials, at a temperature between about 225 C. and about 300 C., and a residence time of more than 1 hour, then (d) continuously extruding said melt, and finally (e) cooling said extruded polymer to form uniformly shaped articles the improvement comprising carrying out step (c) at an absolute pressure of between about 31 mm. and about mm. of Hg, subjecting said melt in step (c) to be sweeping fiow of steam and maintaining the partial pressure of said steam in the vapor above said melt at above about 30 mm.

so that the extractables content of said polycaprm amide at the end of said polymerization is below 3.5% by weight, thermal degradation of the polymer in the melt is minimized to within 3 units of the theoretical difierence of ends, and the rate viscosity increase of the polymer melt levels out after less than 4 hours to less than 10 FAV units per hour.

2. The process of claim 1 wherein said viscosity increase is less than 8 FAV units after less than 3 hours, and thermal degradation is minimized to within 3 units of the theoretical difference of ends.

3. The process of claim 1 wherein said partial pressure of steam is maintained by sweeping steam overhead and counter to the flow of said polymer melt.

References Cited UNITED STATES PATENTS 2,731,081 1/ 1956 Mayner 260-78 L 2,904,109 9/1959 Malm 159-44 X 2,867,805 1/1959 Ludewig 260-78 L 3,090,773 3/ 1963 Papero et a1 260-78 L 3,449,220 6/1969 Geisler et a1 260-78 L X 3,558,567 1/ 1971 Twilley et al 260-78 L 3,679,635 7 7/1972 Portus 260-78 L X 3,386,967 6/ 1968 Twilley 260-78 L HOWARD E. SCI-IAIN, Primary Examiner US. Cl. X.R. 260-78 A 

